The present invention relates to magneto-optic eddy current imaging. More particularly, the invention relates to a magneto-optic eddy current imaging apparatus and method.
Instrumentation has recently been developed which opens up a new area of non-destructive inspection technology known as magneto-optic eddy current imaging. This technology makes it possible to generate a real-time image of defects in both metallic and non-metallic structures. For example, defects, such as fatigue cracks and corrosion in ferrous and non-ferrous metals, or cracks and delamination in non-metallic structures. This class of instrumentation utilizes a sensor having a large Faraday magneto-optic effect (1845). Optically active materials exhibiting this effect will rotate the plane of polarization of polarized light passing through it as a function of the applied, or developed, normal magnetic field. A double pass reflective configuration utilizing a non-magnetic dielectric mirror is known in the art. This is typically the way the magneto-optic crystal is used in an instrument. On each pass of the polarized light through the crystal, regardless of direction, the initial polarization is rotated in the same direction as determined by the magnetic field, thus increasing the sensitivity. In this class of instrument an alternating magnetic field induces a uniform flow of eddy currents into the object which are disrupted in the area of a crack, or other defect. This, in turn, creates secondary magnetic fields normal to the induced eddy currents that flow in the sensor. Since the eddy current is parallel to the plane of the sensor, the magnetic field is normal, or perpendicular to it. These low intensity local magnetic fields cause a local rotation of the plane of polarization of the light passing through the sensor. It is these local rotations of the polarization that cause a spatial image to be formed. The image thus formed is latent until the light is passed through an analyzer which converts the polarization to intensity modulation.
A problem arises, since the class of materials which exhibit a high magneto-optic effect resulting in the production of an image of the normal, or perpendicular, magnetic field perturbed by the crack, or defect, is obscured by a highly visible background. The background results from the magnetic wall domains, which are a fundamental part of the material. One reason for the reduced visibility of the cracks, or defects, is that the aforedescribed class of magnetic materials has many small domains which are visible to the eye. When the image is viewed directly, particularly under magnification, or with the aid of a video camera, the resulting wall-domains give the appearance of cracked paint, or a dry lake bed. The wall domains may resemble small cracks, thus making the detection of such cracks difficult. On the other hand, long cracks will pass through a number of domains, which can cause a distortion in their shape.
The principal object of the invention is to provide magneto-optic eddy current imaging apparatus which functions efficiently, effectively and reliably to produce clear images of defects in a structure test object.
An object of the invention is to provide magneto-optic eddy current imaging apparatus which eliminates the undesired background of a two-dimensional magneto-optically generated image of a defect in a test object.
Another object of the invention is to provide a magneto-optic eddy current imaging method which is efficient, effective and reliable in producing clear images of defects in a structure test object.
Still another object of the invention is to provide a magneto-optic eddy current imaging method which eliminates the undesired background of a two-dimensional magneto-optically generated image of a defect in a test object.
Yet another object of the invention is to provide an apparatus and a method for magneto-optic eddy current imaging which eliminates the undesired background which obscures an indication of defects in a ferrous or non-ferrous metal or non-metallic object and thereby produces a clear image of such defects.
Another object of the invention is to provide an apparatus and a method for magneto-optic eddy current imaging which blanks the magnetic wall domains in the output of a processed video display, thereby reducing and essentially eliminating background interference effects as seen by a viewer, whereby the enhanced image enables the user to detect defects such as, for example, cracks, corrosion and delamination, not previously visible, and enables the attainment of the detection very rapidly.
Still another object of the invention is to provide an apparatus and a method for magneto-optic eddy current imaging which may produce a signal to noise that is, image to background, which is sufficient to permit automatic detection, using pattern recognition techniques, thereby making unmanned monitoring of defects, such as cracks and corrosion, feasible.